Specified the modification time output format in the sitemap

[mhrstmnn]

Specified the modification time output format of the lastmod tag in the sitemap accordingly to:

• Googles Search Documentation
• Sitemaps.org

[mhrstmnn]

I also changed the XML output format and cleaned up some whitespace

[bnomei]

thanks for the pr.

i will fix the bug with the date format in sitemaps. but it needs a little bit more code to support all inputs.

for whitespace formating i will do myself if i find time to verify. github just shows me all lines changed so i have to use a local diff tool.